Method and apparatus for accessing channel in wlan system

ABSTRACT

The present invention relates to a wireless communication system, and more specifically, disclosed are a method and an apparatus for accessing a channel in a WLAN system. A method for accessing the channel from a station (STA) in the wireless communication system, according to one embodiment of the present invention, comprises the steps of: receiving from an access point (AP) setting information with respect to at least one slot allowing channel access by the STA; and beginning the channel access from a slot boundary of the at least one slot, wherein the channel access may begin without a clear channel assessment (CCA).

FIELD OF THE INVENTION

The present invention relates to a wireless communication system, andmore particularly to a method and apparatus for performing channelaccess in a wireless LAN system.

BACKGROUND ART

Various wireless communication technologies systems have been developedwith rapid development of information communication technologies. WLANtechnology from among wireless communication technologies allowswireless Internet access at home or in enterprises or at a specificservice provision region using mobile terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), etc. on the basis of Radio Frequency (RF) technology.

In order to obviate limited communication speed, one of thedisadvantages of WLAN, the recent technical standard has proposed anevolved system capable of increasing the speed and reliability of anetwork while simultaneously extending a coverage region of a wirelessnetwork. For example, IEEE 802.11n enables a data processing speed tosupport a maximum high throughput (HT) of 540 Mbps. In addition,Multiple Input and Multiple Output (MIMO) technology has recently beenapplied to both a transmitter and a receiver so as to minimizetransmission errors as well as to optimize a data transfer rate.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Machine to Machine (M2M) communication technology has been discussed asnext generation communication technology. A technical standard forsupporting M2M communication in IEEE 802.11 WLAN has been developed asIEEE 802.11ah. M2M communication may sometimes consider a scenariocapable of communicating a small amount of data at low speed in anenvironment including a large number of devices.

Communication in the WLAN system is performed in a medium shared by alldevices. If the number of devices as in the case of M2M communicationincreases, consumption of a long time for channel access of a singledevice may unavoidably deteriorate the entire system throughput, and mayprevent power saving of the respective devices.

An object of the present invention is to provide a new channel accessmethod for reducing not only a duration time consumed for channel accessbut also power consumption of the device.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for performing channel access by a station (STA) of a wirelesscommunication system including: receiving configuration informationregarding at least one slot in which channel access of the STA isallowed, from an access point (AP); and starting channel access at aslot boundary of the at least one slot, wherein the channel accessstarts without performing Clear Channel Assessment (CCA).

In accordance with another aspect of the present invention, a station(STA) device configured to perform channel access in a wirelesscommunication system includes: a transceiver; and a processor, whereinthe processor is configured to received, through the transceiver,configuration information regarding at least one slot in which channelaccess of the STA is allowed, from an access point (AP), and to startchannel access at a slot boundary of the at least one slot, wherein thechannel access starts without performing Clear Channel Assessment (CCA).

The following description may be commonly applied to the embodiments ofthe present invention.

The configuration information regarding the at least one slot mayfurther include specific information indicating whether TransmissionOpportunity (TXOP) of the STA is allowed to overlap the slot boundary.If the TXOP is not allowed to overlap the slot boundary, the channelaccess may start without performing the CCA.

The TXOP may be obtained in one slot duration from among the at leastone slot.

The STA may be switched from a doze state to an awake state.

The CCA may be performed until a frame sequence for the STA to configurea network allocation vector (NAV), or until a period of time equal to aProbeDelay value has transpired.

The slot boundary may be a time point at which channel access of the STAis allowed.

The slot may be an interval between two contiguous time points.

The time point at which the channel access may be allowed is a targetawake time of the STA.

The starting the channel access may include transmitting a frame throughcontention.

The frame may be an NDP Power Save (PS)-Poll frame.

A plurality of slots may be configured during a beacon interval.

The configuration information of the at least one slot may be providedthrough a beacon frame.

The configuration information of the at least one slot may beconfiguration information of a time duration in which restricted channelaccess of the STA is allowed.

The configuration information of the at least one slot may includeinformation for allocating the channel access duration allowed to a STAgroup including the STA on a slot basis.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Effects of the Invention

As is apparent from the above description, the embodiments of thepresent invention provide a new channel access method so that a methodand apparatus for reducing not only a time consumed for channel accessbut also power consumption of the device can be provided.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

FIG. 2 exemplarily shows an IEEE 802.11 system according to anotherembodiment of the present invention.

FIG. 3 exemplarily shows an IEEE 802.11 system according to stillanother embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a WLAN system.

FIG. 5 is a flowchart illustrating a link setup process for use in theWLAN system.

FIG. 6 is a conceptual diagram illustrating a backoff process.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

FIG. 9 is a conceptual diagram illustrating a power managementoperation.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof a station (STA) having received a Traffic Indication Map (TIM).

FIG. 13 is a conceptual diagram illustrating a group-based AID.

FIG. 14 is a conceptual diagram illustrating a PS-Poll mechanism.

FIG. 15 is a conceptual diagram illustrating an Unscheduled-AutomaticPower Save Delivery (U-APSD) mechanism.

FIG. 16 is a conceptual diagram illustrating a legacy Clear ChannelAssessment (CCS) operation in a hidden node environment.

FIG. 17 is a conceptual diagram illustrating a channel access operationfor use in the case in which a target awake time is established.

FIG. 18 is a conceptual diagram illustrating an exemplary target awaketime information element format according to an exemplary embodiment.

FIG. 19 is a conceptual diagram illustrating a target awake timeinterval according to an exemplary embodiment.

FIG. 20 is a conceptual diagram illustrating another example of a targetawake time information element format according to an exemplaryembodiment.

FIG. 21 is a flowchart illustrating a channel access operation for usein the case in which a plurality of target awake times is established.

FIG. 22 is a conceptual diagram illustrating an NDP frame formataccording to an exemplary embodiment.

FIG. 23 is a flowchart illustrating a channel access method according toan exemplary embodiment.

FIG. 24 is a block diagram illustrating a radio frequency (RF) deviceaccording to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communication)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). Forclarity, the following description focuses on IEEE 802.11 systems.However, technical features of the present invention are not limitedthereto.

WLAN System Structure

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by mutual operations of the components. A BasicService Set (BSS) may correspond to a basic constituent block in an IEEE802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and two STAsare included in each of the BSSs (i.e. STA1 and STA2 are included inBSS1 and STA3 and STA4 are included in BSS2). An ellipse indicating theBSS in FIG. 1 may be understood as a coverage area in which STAsincluded in the corresponding BSS maintain communication. This area maybe referred to as a Basic Service Area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In IEEE 802.11 LAN, the most basic type of BSS is an Independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. Such a type of LAN is notprescheduled and may be configured when the LAN is necessary. This maybe referred to as an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAis switched on or off or the STA enters or leaves the BSS region. TheSTA may use a synchronization process to join the BSS. To access allservices of a BSS infrastructure, the STA should be associated with theBSS. Such association may be dynamically configured and may include useof a Distribution System Service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a Distribution System (DS), a Distribution SystemMedium (DSM), and an Access Point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by PHYperformance. In some cases, such restriction of the distance may besufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are interconnected.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a Wireless Medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STAT and STA4) to access the DS. Moreover, since all APscorrespond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not always beidentical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an ExtendedService Set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to as an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a Logical Link Control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One or more IBSSs orESS networks may be physically located in the same space as one or moreESS networks. This may correspond to an ESS network form in the case inwhich an ad-hoc network operates in a location in which an ESS networkis present, the case in which IEEE 802.11 networks of differentorganizations physically overlap, or the case in which two or moredifferent access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as laptop computers or mobile phones,handled directly by users. In FIG. 4, STAT, STA3, and STA4 correspond tothe non-AP STAs and STA2 and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment(UE), a Mobile Station (MS), a mobile terminal, or a Mobile SubscriberStation (MSS). The AP is a concept corresponding to a Base Station (BS),a Node-B, an evolved Node-B (e-NB), a Base Transceiver System (BTS), ora femto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow an STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

The link setup process is described referring to FIG. 5.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 5 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which AP (AccessPoint) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be an STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning An STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives a beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning,active scanning is more advantageous than passive scanning in terms ofdelay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process to clearly distinguish theauthentication process from the security setup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a QoS map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on an(Extensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

WLAN Evolution

In order to obviate limitations in WLAN communication speed, IEEE802.11n has recently been established as a communication standard. IEEE802.11n aims to increase network speed and reliability as well as toextend a coverage region of the wireless network. In more detail, IEEE802.11n supports a High Throughput (HT) of a maximum of 540 Mbps, and isbased on MIMO technology in which multiple antennas are mounted to eachof a transmitter and a receiver.

With the widespread use of WLAN technology and diversification of WLANapplications, there is a need to develop a new WLAN system capable ofsupporting a higher throughput (HT) higher than a data processing speedsupported by IEEE 802.11n. The next generation WLAN system forsupporting Very High Throughput (VHT) is the next version (for example,IEEE 802.11ac) of the IEEE 802.11n WLAN system, and is one of IEEE802.11 WLAN systems recently proposed to support a data process speed of1 Gbps or more at a MAC SAP (Medium Access Control Service AccessPoint).

In order to efficiently utilize a radio frequency (RF) channel, the nextgeneration WLAN system supports MU-MIMO (Multi User Multiple InputMultiple Output) transmission in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, a technology for supporting WLAN system operations inwhitespace has recently been discussed. For example, a technology forintroducing the WLAN system in whitespace (TV WS) such as an idlefrequency band (for example, 54˜698 MHz band) abandoned because of thetransition to digital TV has been discussed under the IEEE 802.11afstandard. However, the above-mentioned information is disclosed forillustrative purposes only, and the whitespace may be a licensed bandcapable of being primarily used only by a licensed user. The licenseduser may be a user who has authority to use the licensed band, and mayalso be referred to as a licensed device, a primary user, an incumbentuser, or the like.

For example, an AP and/or STA operating in the whitespace (WS) mustprovide a function for protecting the licensed user. For example,assuming that the licensed user such as a microphone has already used aspecific WS channel acting as a divided frequency band on regulation ina manner that a specific bandwidth of the WS band is occupied, the APand/or STA cannot use the frequency band corresponding to thecorresponding WS channel so as to protect the licensed user. Inaddition, the AP and/or STA must stop using the corresponding frequencyband under the condition that the licensed user uses a frequency bandused for transmission and/or reception of a current frame.

Therefore, the AP and/or STA must determine whether to use a specificfrequency band of the WS band. In other words, the AP and/or STA mustdetermine the presence or absence of an incumbent user or a licenseduser in the frequency band. The scheme for determining the presence orabsence of the incumbent user in a specific frequency band is referredto as a spectrum sensing scheme. An energy detection scheme, a signaturedetection scheme and the like may be used as the spectrum sensingmechanism. The AP and/or STA may determine that the frequency band isbeing used by an incumbent user if the intensity of a received signalexceeds a predetermined value, or when a DTV preamble is detected.

M2M (Machine to Machine) communication technology has been discussed asnext generation communication technology. A technical standard forsupporting M2M communication has been developed as IEEE 802.11 ah in theIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines, or may also be referred to asMachine Type Communication (MTC) or Machine To Machine (M2M)communication. In this case, the machine may be an entity that does notrequire direct handling and intervention of a user. For example, notonly a meter or vending machine including a RF module, but also a userequipment (UE) (such as a smartphone) capable of performingcommunication by automatically accessing the network without userintervention/handling may be an example of such machines. M2Mcommunication may include Device-to-Device (D2D) communication andcommunication between a device and an application server, etc. Asexemplary communication between the device and the application server,communication between a vending machine and an application server,communication between the Point of Sale (POS) device and the applicationserver, and communication between an electric meter, a gas meter or awater meter and the application server. M2M-based communicationapplications may include security, transportation, healthcare, etc. Inthe case of considering the above-mentioned application examples, M2Mcommunication has to support a method for sometimestransmitting/receiving a small amount of data at low speed under anenvironment including a large number of devices.

In more detail, M2M communication must support a large number of STAs.Although the current WLAN system assumes that one AP is associated witha maximum of 2007 STAs, various methods for supporting other cases inwhich many more STAs (e.g., about 6000 STAs) are associated with one APhave recently been discussed in M2M communication. In addition, it isexpected that many applications for supporting/requesting a low transferrate will participate in M2M communication. In order to smoothly supportmany STAs, the WLAN system may recognize the presence or absence of datato be transmitted to the STA on the basis of a TIM (Traffic Indicationmap), and various methods for reducing the bitmap size of the TIM haverecently been discussed. In addition, it is expected that much trafficdata having a very long transmission/reception interval will participatein M2M communication. For example, in M2M communication, a very smallamount of data (e.g., electric/gas/water metering) needs to betransmitted at long intervals (for example, every month). Therefore,although the number of STAs associated with one AP increases in the WLANsystem, many developers and companies are conducting intensive researchinto a WLAN system which can efficiently support the case in which thereare a very small number of STAs, each of which has a data frame to bereceived from the AP during one beacon period.

As described above, WLAN technology is rapidly developing, and not onlythe above-mentioned exemplary technologies but also other technologiessuch as a direct link setup, improvement of media streaming throughput,high-speed and/or support of large-scale initial session setup, andsupport of extended bandwidth and operation frequency, are beingintensively developed.

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of MAC(Medium Access Control) is a Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism isreferred to as a Distributed Coordination Function (DCF) of IEEE 802.11MAC, and basically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start transmission, establishes a delay time (forexample, a random backoff period) for medium access, and attempts tostart frame transmission after waiting for a predetermined time. Throughapplication of a random backoff period, it is expected that multipleSTAs will attempt to start frame transmission after waiting fordifferent times, resulting in minimum collision.

In addition, the IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 6 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 6. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count is a pseudo-random integer,and may be set to one of 0 to CW values. In this case, CW refers to aContention Window parameter value. Although an initial value of the CWparameter is denoted by CWmin, the initial value may be doubled in caseof a transmission failure (for example, in the case in which ACK of thetransmission frame is not received). If the CW parameter value isdenoted by CWmax, CWmax is maintained until data transmission issuccessful, and at the same time it is possible to attempt to start datatransmission. If data transmission was successful, the CW parametervalue is reset to CWmin. Preferably, CW, CWmin, and CWmax are set to2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is in the occupied state,the countdown stops and waits for a predetermined time. If the medium isin the idle state, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of data transmission failure.In this case, each of STA4 and STA5 increases the CW value two times,and STA4 or STA5 may select a random backoff count value and thenperform countdown. Meanwhile, STA1 waits for a predetermined time whilethe medium is in the occupied state due to transmission of STA4 andSTA5. In this case, if the medium is in the idle state, STA1 waits forthe DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For virtual carrier sensing,MAC of the WLAN system can utilize a Network Allocation Vector (NAV). Inmore detail, by means of the NAV value, the AP and/or STA, each of whichcurrently uses the medium or has authority to use the medium, may informanother AP and/or another STA of the remaining time for which the mediumis available. Accordingly, the NAV value may correspond to a reservedtime in which the medium will be used by the AP and/or STA configured totransmit the corresponding frame. An STA having received the NAV valuemay prohibit medium access (or channel access) during the correspondingreserved time. For example, NAV may be set according to the value of a‘duration’ field of the MAC header of the frame.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be given with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 7( a) exemplarily shows the hidden node. In FIG. 7( a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 7( a), STA C may determine that the medium is in the idle statewhen performing carrier sensing before transmitting data to STA B, underthe condition that STA A transmits information to STA B. Sincetransmission of STA A (i.e., occupied medium) may not be detected at thelocation of STA C, it is determined that the medium is in the idlestate. In this case, STA B simultaneously receives information of STA Aand information of STA C, resulting in collision. Here, STA A may beconsidered a hidden node of STA C.

FIG. 7( b) exemplarily shows an exposed node. In FIG. 7( b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS (request to send) and CTS (clear to send).RTS/CTS between two STAs may be overheard by peripheral STA(s), suchthat the peripheral STA(s) may consider whether information iscommunicated between the two STAs. For example, if STA to be used fordata transmission transmits the RTS frame to the STA having receiveddata, the STA having received data transmits the CTS frame to peripheralSTAs, and may inform the peripheral STAs that the STA is going toreceive data.

FIG. 8( a) exemplarily shows a method for solving problems of the hiddennode. In FIG. 8( a), it is assumed that each of STA A and STA C is readyto transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 8( b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe transmitted can transmit a CTS. STA C receives only the RTS and doesnot receive the CTS of STA A, such that it can be recognized that STA Ais located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the reception(Rx) state and the transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-restricted STA (i.e., anSTA operated by a battery). Therefore, if STA maintains the Rx standbymode so as to persistently sense the channel, power is inefficientlyconsumed without special advantages in terms of WLAN throughput. Inorder to solve the above-mentioned problem, the WLAN system supports apower management (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning

Power consumption is reduced in proportion to a specific time in whichthe STA stays in the sleep state, such that the STA operation time isincreased in response to the reduced power consumption. However, it isimpossible to transmit or receive a frame in the sleep state, such thatthe STA cannot mandatorily operate for a long period of time. If thereis a frame to be transmitted to the AP, the STA operating in the sleepstate is switched to the awake state, such that it can transmit/receivethe frame in the awake state. On the other hand, if the AP has a frameto be transmitted to the STA, the sleep-state STA is unable to receivethe frame and cannot recognize the presence of a frame to be received.Accordingly, STA may need to periodically switch to the awake state inorder to recognize the presence or absence of a frame to be transmittedto the STA (or in order to receive a signal indicating the presence ofthe frame on the assumption that the presence of the frame to betransmitted to the STA is decided).

FIG. 9 is a conceptual diagram illustrating a power management (PM)operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at intervals of a predetermined time period in steps (S211,S212, S213, S214, S215, S216). The beacon frame includes a TIMinformation element. The TIM information element includes bufferedtraffic regarding STAs associated with the AP 210, and includes specificinformation indicating that a frame is to be transmitted. The TIMinformation element includes a TIM for indicating a unicast frame and aDelivery Traffic Indication Map (DTIM) for indicating a multicast orbroadcast frame.

AP 210 may transmit a DTIM once whenever the beacon frame is transmittedthree times. Each of STA1 220 and STA2 222 is operated in the PS mode.Each of STA1 220 and STA2 222 is switched from the sleep state to theawake state every wakeup interval, such that STA1 220 and STA2 222 maybe configured to receive the TIM information element transmitted by theAP 210. Each STA may calculate a switching start time at which each STAmay start switching to the awake state on the basis of its own localclock. In FIG. 9, it is assumed that a clock of the STA is identical toa clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state in step S221 when AP 210 first transmits the beacon frame instep S211. STA1 220 receives the beacon frame, and obtains the TIMinformation element. If the obtained TIM element indicates the presenceof a frame to be transmitted to STA1 220, STA1 220 may transmit a PowerSave-Poll (PS-Poll) frame, which requests the AP 210 to transmit theframe, to the AP 210 in step S221 a. The AP 210 may transmit the frameto STA 1 220 in response to the PS-Poll frame in step S231. STA1 220having received the frame is re-switched to the sleep state, andoperates in the sleep state.

When AP 210 secondly transmits the beacon frame, a busy medium state inwhich the medium is accessed by another device is obtained, the AP 210may not transmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time in step S212. In this case,although STA1 220 is switched to the awake state in response to thebeacon interval, it does not receive the delay-transmitted beacon frameand, as such, re-enters the sleep state in step S222.

When AP 210 thirdly transmits the beacon frame, the corresponding beaconframe may include a TIM element denoted by DTIM. However, since the busymedium state is given, AP 210 transmits the beacon frame at a delayedtime in step S213. STA1 220 is switched to the awake state in responseto the beacon interval, and may obtain a DTIM through the beacon frametransmitted by the AP 210. It is assumed that DTIM obtained by STAT 220does not have a frame to be transmitted to STA1 220 but there is a framefor another STA. In this case, STA1 220 confirms the absence of a frameto be received in the STA1 220, and re-enters the sleep state, such thatthe STA1 220 may operate in the sleep state. After the AP 210 transmitsthe beacon frame, the AP 210 transmits the frame to the correspondingSTA in step S232.

AP 210 fourthly transmits the beacon frame in step S214. However, it isimpossible for STA1 220 to obtain information regarding the presence ofbuffered traffic associated with the STA1 220 through double receptionof a TIM element, such that the STA1 220 may adjust the wakeup intervalfor receiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by AP 210, the wakeup intervalvalue of the STA1 220 may be adjusted. In this example, STA1 220, whichhas been switched to receive a TIM element every beacon interval, may beswitched to another operation state in which STA1 220 can awake from thesleep state once every three beacon intervals. Therefore, when AP 210transmits a fourth beacon frame in step S214 and transmits a fifthbeacon frame in step S215, STA1 220 maintains the sleep state such thatit cannot obtain the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame in step S216, STA1 220 isswitched to the awake state and operates in the awake state, such thatthe STA1 220 is unable to obtain the TIM element contained in the beaconframe in step S224. The TIM element is a DTIM indicating the presence ofa broadcast frame, such that STA1 220 does not transmit the PS-Pollframe to the AP 210 and may receive a broadcast frame transmitted by theAP 210 in step S234. In the meantime, the wakeup interval of STA2 230may be longer than a wakeup interval of STA1 220. Accordingly, STA2 230enters the awake state at a specific time S215 where the AP 210 fifthlytransmits the beacon frame, such that the STA2 230 may receive the TIMelement in step S241. STA2 230 recognizes the presence of a frame to betransmitted to the STA2 230 through the TIM element, and transmits thePS-Poll frame to the AP 210 so as to request frame transmission in stepS241 a. AP 210 may transmit the frame to STA2 230 in response to thePS-Poll frame in step S233.

In order to operate/manage the power save (PS) mode shown in FIG. 9, theTIM element may include either a TIM indicating the presence or absenceof a frame to be transmitted to the STA, or a DTIM indicating thepresence or absence of a broadcast/multicast frame. DTIM may beimplemented through field setting of the TIM element.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof the STA having received a Traffic Indication Map (TIM).

Referring to FIG. 10, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 10, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be given with reference toFIG. 11.

The STA operations of FIG. 11 in which the STA is switched from thesleep state to the awake state, receives a TIM from the AP, andtransmits the PS-Poll frame to the AP through contention are identicalto those of FIG. 10. If the AP having received the PS-Poll frame doesnot prepare a data frame during the SIFS time, the AP may transmit theACK frame to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of contention. STAmay transmit the ACK frame indicating successful reception of a dataframe to the AP, and may then be shifted to the sleep state.

FIG. 12 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintain the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

TIM Structure

In the operation and management method of the Power save (PS) mode basedon the TIM (or DTIM) protocol shown in FIGS. 9 to 12, STAs may determinethe presence or absence of a data frame to be transmitted for the STAsthrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan Association Identifier (AID) to be allocated when an STA isassociated with an AP.

AID is used as a unique ID of each STA within one BSS. For example, AIDfor use in the current WLAN system may have a value of 1 to 2007. In thecase of the current WLAN system, 14 bits for AID may be allocated to aframe transmitted by AP and/or STA. Although the AID value may beassigned a maximum value of 16383, the values of 2008 to 16383 are setto reserved values.

The TIM element according to legacy definition is inappropriate for M2Mcommunication through which many STAs (for example, at least 2007 STAs)are associated with one AP. If the conventional TIM structure isextended without any change, the TIM bitmap size excessively increases,such that it is impossible to support the extended TIM structure usingthe legacy frame format, and the extended TIM structure is inappropriatefor M2M communication in which a low transfer rate is considered. Inaddition, it is expected that there will be a very small number of STAseach having an Rx data frame during one beacon period. Therefore,according to exemplary application of the above-mentioned M2Mcommunication, it is expected that the TIM bitmap size is increased andmost bits are set to zero (0), such that there is needed a technologycapable of efficiently compressing such bitmap.

In the legacy bitmap compression technology, successive values (each ofwhich is set to zero) of 0 are omitted from a header part of bitmap, andthe omitted result may be defined as an offset (or start point) value.However, although STAs each including the buffered frame is small innumber, if there is a high difference between AID values of respectiveSTAs, compression efficiency is not high. For example, assuming that theframe to be transmitted to only a first STA having an AID of 10 and asecond STA having an AID of 2000 is buffered, the length of a compressedbitmap is set to 1990, and the remaining parts other than both edgeparts are assigned zero (0). If STAs associated with one AP is small innumber, inefficiency of bitmap compression does not cause seriousproblems. However, if the number of STAs associated with one APincreases, such inefficiency may deteriorate overall system throughput.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmittedusing the AIDs. A designated group ID (GID) is allocated to each group.AIDs allocated on the basis of such grouping will hereinafter bedescribed with reference to FIG. 13.

FIG. 13( a) is a conceptual diagram illustrating a group-based AID. InFIG. 13( a), some bits located at the front part of the AID bitmap maybe used to indicate a group ID (GID). For example, it is possible todesignate four GIDs using the first two bits of an AID bitmap. If atotal length of the AID bitmap is denoted by N bits, the first two bits(B1 and B2) may represent a GID of the corresponding AID.

FIG. 13( b) is a conceptual diagram illustrating a group-based AID. InFIG. 13( b), a GID may be allocated according to the position of theAID. In this case, AIDs having the same GID may be represented by offsetand length values. For example, if GID 1 is denoted by Offset A andLength B, this means that AIDs (A˜A+B−1) on bitmap are respectively setto GID 1. For example, FIG. 13( b) assumes that AIDs (1˜N4) are dividedinto four groups. In this case, AIDs contained in GID 1 are denoted by1˜N1, and the AIDs contained in this group may be represented by Offset1 and Length N1. AIDs contained in GID 2 may be represented by Offset(N1+1) and Length (N2−N1+1), AIDs contained in GID 3 may be representedby Offset (N2+1) and Length (N3−N2+1), and AIDs contained in GID 4 maybe represented by Offset (N3+1) and Length (N4−N3+1).

In case of using the aforementioned group-based AIDs, channel access isallowed in a different time interval according to individual GIDs, theproblem caused by the insufficient number of TIM elements compared witha large number of STAs can be solved and at the same time data can beefficiently transmitted/received. For example, during a specific timeinterval, channel access is allowed only for STA(s) corresponding to aspecific group, and channel access to the remaining STA(s) may berestricted. A predetermined time interval in which access to onlyspecific STA(s) is allowed may also be referred to as a RestrictedAccess Window (RAW).

Channel access based on GID will hereinafter be described with referenceto FIG. 13( c). If AIDs are divided into three groups, the channelaccess mechanism according to the beacon interval is exemplarily shownin FIG. 13( c). A first beacon interval (or a first RAW) is a specificinterval in which channel access to an STA corresponding to an AIDcontained in GID 1 is allowed, and channel access of STAs contained inother GIDs is disallowed. For implementation of the above-mentionedstructure, a TIM element used only for AIDs corresponding to GID 1 iscontained in a first beacon frame and a TIM element used only for AIDscorresponding to GID 2 is contained in a second beacon frame.Accordingly, only channel access to an STA corresponding to the AIDcontained in GID 2 is allowed during a second beacon interval (or asecond RAW) during a second beacon interval (or a second RAW). A TIMelement used only for AIDs having GID 3 is contained in a third beaconframe, such that channel access to an STA corresponding to the AIDcontained in GID 3 is allowed using a third beacon interval (or a thirdRAW). A TIM element used only for AIDs each having GID 1 is contained ina fourth beacon frame, such that channel access to an STA correspondingto the AID contained in GID 1 is allowed using a fourth beacon interval(or a fourth RAW). Thereafter, only channel access to an STAcorresponding to a specific group indicated by the TIM contained in thecorresponding beacon frame may be allowed in each of beacon intervalssubsequent to the fifth beacon interval (or in each of RAWs subsequentto the fifth RAW).

Although FIG. 13( c) exemplarily shows that the order of allowed GIDs isperiodical or cyclical according to the beacon interval, the scope orspirit of the present invention is not limited thereto. That is, onlyAID(s) contained in specific GID(s) may be contained in a TIM element,such that channel access to STA(s) corresponding to the specific AID(s)is allowed during a specific time interval (for example, a specificRAW), and channel access to the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks, and channel access toSTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having any one of the remaining values other than ‘0’ may beallowed. Therefore, if a large TIM is divided into small blocks/groups,STA can easily maintain TIM information, and blocks/groups may be easilymanaged according to class, QoS or usage of the STA. Although FIG. 13exemplarily shows a 2-level layer, a hierarchical TIM structurecomprised of two or more levels may be configured. For example, a totalAID space may be divided into a plurality of page groups, each pagegroup may be divided into a plurality of blocks, and each block may bedivided into a plurality of sub-blocks. In this case, according to theextended version of FIG. 13( a), first N1 bits of AID bitmap mayrepresent a page ID (i.e., PID), the next N2 bits may represent a blockID, the next N3 bits may represent a sub-block ID, and the remainingbits may represent the position of STA bits contained in a sub-block.

In the examples of the present invention, various schemes for dividingSTAs (or AIDs allocated to respective STAs) into predeterminedhierarchical group units, and managing the divided result may be appliedto the embodiments, however, the group-based AID allocation scheme isnot limited to the above examples.

Improved Channel Access Method

FIG. 14 is a conceptual diagram illustrating a PS-Poll mechanism. Inmore detail, FIG. 14 is a detailed example of the PS-Poll mechanismshown in FIG. 11.

As described above, the STA may recognize the presence or absence ofdata to be transmitted from the AP to the STA through a TIM element ofthe beacon. The STA having recognized the presence of data to betransmitted thereto, may transmit the PS-Poll frame to the AP so as torequest data (i.e., DL data) from the AP. The AP having received thePS-Poll frame may transmit data to the STA through contention. In moredetail, the AP configured to attempt to transmit data may transmit theRTS frame to the STA having received the data. The STA to be used fordata reception transmits the CTS frame so that it can indicate that theSTA is ready to receive data. Therefore, the AP may transmit a dataframe to the STA, and may receive the ACK frame. In this case, the APmay transmit only one Physical layer Service Data Unit (PSDU) to the STAonce. Therefore, if there is a large amount of data to be sent from theAP to the STA, the AP must transmit data through contention in responseto a new PS-Poll from the STA, so that data transmission may beinefficiently carried out.

FIG. 15 is a conceptual diagram illustrating an Unscheduled-AutomaticPower Save Delivery (U-APSD) mechanism.

Referring to FIG. 15, according to the U-APSD (Unscheduled-AutomaticPower Save Delivery) mechanism, in order to use a U-APSD SP, the STA caninform the AP of a requested transmission duration and the AP cantransmit a frame to the STA for the SP. According to the U-APSDmechanism, the STA can simultaneously receive a plurality of PSDUs fromthe AP.

Referring to FIG. 15, the STA may recognize the presence of data to betransmitted from the STA to the AP through the TIM element of thebeacon. The STA can recognize that the AP has data to be sent theretothrough a TIM element of a beacon. Then, the STA can request the AP totransmit while signaling to the AP that the SP of the STA starts bytransmitting a trigger frame to the AP at a desired time. The AP cantransmit ACK as a response to the trigger frame. Subsequently, the APcan transmit an RTS to the STA through contention, receive a CTS framefrom the STA and then transmit data to the STA. Here, the datatransmitted by the AP can be composed of one or more data frames. Whenthe AP sets the end of service period (EOSP) of the last data frame to 1and transmits the last data frame to the STA, the STA can recognize theEOSP and end the SP. Accordingly, the STA can transmit ACK indicatingthat the STA has successfully received the data. According to the U-APSDmechanism, the STA can start the SP thereof at a desired time to receivedata and receive multiple data frames within a single SP, therebyachieving efficient data reception.

In the meantime, as shown in FIGS. 14 and 15, an exchange of the RTS/CTSframes during data Tx/Rx times so as to prevent the occurrence of thehidden node problem may cause a large amount of signaling overhead toboth of the data transmission/reception sides. In addition, as shown inFIG. 15, a long period of time from a start time, at which the STAtransmits the trigger frame and requests data transmission to the AP, toan end time, at which the AP prepares for data to be sent to the STA,transmits/receives the RTS/CTS frames through contention for datatransmission and finally transmits data, is consumed such that the STAconsumes a large amount of power.

For example, under the hidden node environment, there may be an STA thatcannot perform overhearing of the PS-Poll frames sent from other STAs,and PS-Poll frames may be simultaneously transferred from a plurality ofSTAs so that there may occur unexpected collision between the PS-Pollframes. Moreover, under the environment in which a large number of STAscan be associated with one AP as in M2M communication, the hidden nodeproblem may more frequently occur. Although the legacy CTS/RTS frameexchange method for solving the hidden node problem is used, powerconsumption caused by transmission/reception (Tx/Rx) of the CTS/RTSframes may cause large load in case of low-power STA, etc. appropriatefor M2M communication.

Improved CCA Operation

The present invention provides a new rule for the CCA operation. In moredetail, the present invention proposes a new CCA rule for use in thecase in which STA is shifted from a doze state (or a sleep state) to anawake state.

In order for the STA operating in the power save (PS) mode to transmituplink traffic, the STA switches from the doze state to the awake stateand then must perform the CCA operation. For example, other STAs performdata Tx/Rx operations at a specific time at which a certain STA switchesfrom the doze state to the awake state, so that the STAs may occupy thechannel. In this case, the awake STA confirms the presence or absence ofTx/Rx operations of other STAs (i.e., performing CCA), and performs theTx operation only when the channel is not occupied, such that Tx/Rxoperation of other STAs can be protected.

In the legacy IEEE 802.11 standard, the CCA rule applied when STA isshifted from the doze state to the awake state is defined as follows. ASTA that is changing from Doze to Awake in order to transmit data shallperform CCA until a frame sequence is detected by which it can correctlyset its NAV, or until a period of time equal to the ProbeDelay hastranspired.

In accordance with the CCA rule, it is necessary to perform CCA untilthe frame sequence capable of correctly establishing the NAV isdetected, and the corresponding time may be limited to a ProbeDelayvalue as necessary.

In this case, NAV is an indicator occupied by each STA. In more detail,in case of both a first case in which a wireless medium is in a busystate due to the CCA function of the STA and a second case in which thewireless medium is not in the busy state, the NAV indicates a timeperiod in which the STA must not initiate data transmission to the WM.The frame sequence may indicate one or more frames, each of which isconfigured to transmit one data unit (e.g., MAC Service Data Unit(MSDU)). If one MSDU is divided into a plurality of fragments andtransmitted through a plurality of frames, the plurality of fragmentshas the same one frame sequence number. ProbeDelay may indicate a delayvalue to be applied prior to the case in which STA is changed from thedoze state to the awake state. For example, ProbeDelay may be set to amaximum PPDU (PLCP Physical Layer Convergence Protocol) transmissiontime. That is, ProbeDelay may be used when the frame sequence in whichNAV is established is not detected.

The above-mentioned legacy CCA rule can prevent transmission collisionbetween STAs without any problem. The following problems may occur inthe evolved system (i.e., IEEE 802.11ah system in which a BSS havinglarge service coverage is introduced.

For example, even when a certain STA performs CCA by detecting a framesequence capable of correctly establishing the NAV and starts datatransmission at a specific time at which an idle channel state isdecided, there may arise the problem in which transmission/reception(Tx/Rx) of other STAs operated in the corresponding channel cannot beprotected in the following hidden node environment.

FIG. 16 is a conceptual diagram illustrating a legacy Clear ChannelAssessment (CCS) operation in the hidden node environment.

In FIG. 16, it is assumed that STA1, STA2, and AP2 belong to the sameBSS, and AP2 belongs to another BSS. In addition, it is also assumedthat STA1 and STA2 are mutual hidden nodes.

As can be seen from FIG. 16, while STA transmits a data frame to AP1,STA1 may be switched from the doze state to the awake state. The awakeSTA1 can receive (or overhear) the beacon frame of the AP2 contained inanother BSS instead of a BSS to which the awake STA1 belongs, and cancorrectly establish the NAV of STA1 on the basis of a parameter (e.g.,duration field) contained in the beacon frame. That is, according to thecurrent CCA rule, when using a frame sequence capable of correctlyestablishing the NAV, CCA is performed before the corresponding framesequence is detected irrespective of a transmission entity of thecorresponding frame sequence. Thereafter, STA1 does not perform datatransmission during the established NAV period. If NAV has expired, STA1may perform channel access through contention (e.g., a backoffoperation). In this case, STA1 may transmit the PS-Poll frame to AP(i.e., AP1) of a BSS to which STA1 belongs so as to perform channelaccess. Therefore, transmission of a data frame of STA1 may collide withtransmission f the PS-Poll frame of STA1. This problem may occur becauseSTA1 acting as a hidden node of STA2 does not receive (or overhear) adata frame of STA2 being occupied a current channel. That is, accordingto the legacy CCA rule, if STA1 can correctly establish its own NAVvalue through an arbitrary frame of another BSS under the hidden nodesituation, there may arise the problem in which transmission of STA2acting as a hidden node of STA1 cannot be protected.

Therefore, the present invention provides a new CCA rule for preventingthe occurrence of the above-mentioned problem. A new CCA rule proposedby the present invention can be defined as follows. A STA that ischanging from Doze to Awake in order to transmit data shall perform CCAuntil a frame sequence in the same BSS is detected by which it cancorrectly set its NAV, or until a period of time equal to the ProbeDelayhas transpired.

CCA must be carried out until the frame sequence capable of correctlyestablishing the NAV is detected and the CCA performing time is limitedto the ProbeDelay value in the same manner as in the legacy CCA rule. Incontrast, according to the new CCA rule, a specific condition in whichthe frame sequence must be a frame sequence belonging to the same BSSmay be added as necessary. That is, CCA must be carried out until NAV iscorrectly established by the frame sequence belonging to the same BSS.

In accordance with the new CCA rule, although STA1 performs overhearingof the beacon frame of AP2 under the situation of FIG. 16, and thebeacon frame corresponds to the frame sequence capable of correctlyestablishing the NAV, the beacon frame does not correspond to the framesequence transmitted in the same BSS as that of STA1, so that STA1 mustcontinuously perform the CCA operation. That is, STA1 continuouslyperforms the CCA operation before receiving the frame sequence that iscapable of correctly establishing the NAV while being transmitted in thesame BSS, so that it is possible for STA1 acting as a hidden node toperform data transmission during transmission of the STA data frame.

Target Awake Time (TAT)

As described above, the legacy CCA rule has been defined to perform CCAwhen STA is switched from the doze state to the awake state. If the STAperforms CCA, associated power consumption may occur. The presentinvention provides a new concept called a target awake time (TAT) toreduce or remove power consumption.

The target awake time (TAT) may indicate a specific value that isallocated and transmitted to STA (s) by the AP. In more detail, thetarget awake time (TAT) may indicate a specific time at which STA(s)operated in the PS mode are switched from the doze state to the awakestate. In addition, STA(s) awakened at the target awake time may beestablished not to perform the CCA operation.

Therefore, a protocol associated with the target awake time (TAT) can bedefined as follows. At the target awake time, (1) STA, that is changingfrom Doze to Awake in order to perform data transmission, performs CCAuntil a period of time equal to the ProbeDelay has transpired; and (2)TXOP (Transmission Opportunity) or transmission within a TXOP shall notextend across a target awake time.

In this case, TXOP is defined as a time interval during which a specificSTA has authority to initiate frame exchange on WM, and may beestablished by a start time and a maximum duration value.

The above-mentioned item (1) may indicate that the awakened STAaccording to the target awake time (TAT) configuration does not performthe CCA operation for NAV configuration. In more detail, the STAswitched from the doze state to the awake state at the target awake time(TAT) does not perform CCA for NAV configuration, and can immediatelyinitiate the backoff process and the channel access operation.

In this case, no limited interpretation must be applied to STA, that isawakened at the target awake time (TAT) performs the CCA operation. Thatis, STA may perform the CCA operation during a predetermined time at atarget awake time (TAT) as necessary. In this case, the predeterminedtime in which the STA performs CCA may be shorter than the ProbeDelaytime.

The above-mentioned item (2) may indicate that anytransmission/reception (Tx/Rx) of STA is not allowed at the target awaketime (TAT). For example, assuming that TXOP is in progress, thecorresponding TXOP may be configured to stop before the target awaketime (TAT). In other words, TXOP may be configured not to overlap withthe target awake time (TAT). In addition, assuming that the target awaketime (TAT) indicates a boundary of time slots to be described later,TXOP may not cross the boundary of the time slots.

The AP may establish a plurality of target awake times (TATs) within thebeacon interval (i.e., a time period to a beacon frame transmission timesubsequent to a single beacon frame transmission time), and may informSTA(s) of the target awake time (TAT) configuration.

FIG. 17 is a conceptual diagram illustrating a channel access operationfor use in the case in which a target awake time is established.

Referring to FIG. 17, if the target awake time (TAT) is established inSTA operating in the PS mode, a doze-state STA may be switched from thetarget awake time (TAT of FIG. 17) to the awake state. Although aplurality of TATs may be established during a single beacon interval,only one TAT is exemplarily shown in FIG. 17 for clarity. The singlebeacon interval may be represented by a time period ranging from atransmission time of a certain beacon frame to a Target BeaconTransmission Time (TBTT). TBTT may correspond to the next beacontransmission (Tx) time.

In accordance with the present invention, STA awakened at TAT does notperform the CCA operation, and may immediately transmit the data frameafter passing through the backoff process. For example, unlike theconventional operation in which STA transitions to the awake state as inSTA2 of FIG. 16 and performs the CCA operation until receiving the framesequence capable of correctly establishing the NAV, STA transitioned tothe awake state at TAT as shown in FIG. 17 may not perform the CCAoperation until reaching a predetermined time corresponding toProbeDelay.

In addition, STA in which TAT is established may be configured in amanner that TXOP does not include TAT. In this case, an underway TXOPmay stop operation prior to a specific time corresponding to TAT. In theexample of FIG. 17, TXOP at which STA performs Tx/Rx operations relatedto another STA (e.g., AP) upon reception of the beacon frame may bestopped prior to TAT.

FIG. 18 is a conceptual diagram illustrating an exemplary target awaketime (TAT) information element format according to an exemplaryembodiment.

Target awake time (TAT) information element (IE) shown in FIG. 18 may betransferred to the STA by AP. A target awake time (IE) may beadditionally contained in the legacy frame such as a beacon frame, aprobe response frame, and an association response frame. Alternatively,the target awake time IE may also be contained in a new format frame fortransmitting the target awake time IE.

In FIG. 18, the element ID field may be set to a specific valueindicating that the corresponding IE is a target awake time (TAT) IE.The Length field may be set to a specific value indicating that thelength of each subsequent field is represented in units of apredetermined unit (e.g., in units of octet. The subsequent fields mayinclude a Target Awake Time Start Offset field, a Target Awake Timeinterval field, target awake STAs at the target awake time #n, or a GID(Target Awake STAs or GID at Target Awake Time #n) field, etc.

The target awake time (TAT) start offset field may indicate a start timeof the target awake time (TAT), and may be set to a specific value bywhich the target awake time is spaced apart from a predeterminedreference time. The predetermined reference time may be TBTT. Forexample, the target awake time start offset may indicate how far thestart time of the target awake time is distant from TBTT. Although FIG.18 shows that the target awake time start offset field is 4 octets long,the scope or spirit of the present invention is not limited thereto, andthe target awake time start offset field may also be defined as adifferent-sized field according to the range of a time start offsetvalue or the like.

The target awake time (TAT) interval field may indicate a time intervalbetween two contiguous target awake times. For example, if a pluralityof TATs is established in a single beacon interval, a time point of afirst target awake time may be decided by the above target awake timestart offset, and a time point of the subsequent target awake time maybe set to a specific time that is spaced apart from the target awaketime start offset value by the target awake time interval value. Forexample, the spacing between the target awake times (TATs) may beequally set to a specific value provided from the target awake timeinterval (TAT).

FIG. 19 is a conceptual diagram illustrating a target awake time (TAT)interval according to an exemplary embodiment.

As can be seen from FIG. 19, the target awake time interval may also beappreciated as a time slot. That is, if a plurality of time points(i.e., a plurality of target awake times (TATs)) at which STA awakes toperform channel access are established, a time interval between the timepoints may be represented by a slot (i.e., a target awake time (TAT)interval). In this case, the target awake time may also be appreciatedas a slot boundary.

As described above, STA, that has awakened at the target awake time(TAT), can immediately perform channel access through a backoff processwithout performing CCA, such that the target awake time (TAT) intervalmay also be denoted by a time period in which channel access isrestrictively allowed for the STA awakened from the target awake time(i.e., channel access of another STA in which TAT is not established isprohibited). Therefore, the time interval (e.g., a time intervalcomposed of three TAT intervals of FIG. 19) including at least onetarget awake time interval may also be denoted by a restricted accesswindow (RAW) according to the present invention. That is, specificinformation indicating the position and length of the target awake timeinterval shown in FIG. 18 may also be understood as RAW configurationinformation, and this information may be decided by AP and signaled tothe STA.

Although FIG. 18 exemplarily shows that the target awake time (TAT)interval field is 4 octets long, the scope or spirit of the presentinvention is not limited thereto, and the target awake time start (TAT)interval field may also be defined as a different-sized field accordingto the range of a time start offset value or the like.

Referring back to FIG. 18, the target awake STAs at a target awake time#n or a GID field (hereinafter referred to as a target awake STA field)thereat may include ID information of TA(s), channel access of which isallowed at Target Awake Time #n. If several target awake times (TATs)are established in a single beacon interval, the target awake time #n(n=1, 2, . . . ) may be sequentially indexed to the TATs.

In addition, the target awake STA field may include a plurality ofsubfields. One subfield may include ID(s) (e.g., AID) of one or moreSTAs, channel access of which is allowed at one TAT. As an example forindicating an identifier (ID) (i.e., the list of ID information ofSTA(s) being channel-accessed at a single TAT #n) of the STA containedin one subfield, the range information (i.e., {start AID, end AID}) ofthe AID being 4 bits long may be used for the above example.

In addition, as shown in FIG. 18, assuming that one subfield from amongthe target awake STA field is 4 octets long and includes N subfields,the length of the target awake STA field may be denoted by 4*n octets.However, the scope or spirit of the present invention is not limitedthereto, the target awake STA field may have different sizes accordingto the number of STAs being channel-accessed at one TAT, the number ofSTAs within one AP, and the ID type (e.g., a complete AID field or apartial AID field) of the STA.

If several STAs are grouped (or paired) into one MU-MIMO group, a groupidentifier (GID) may be contained in the target awake STA field. Forexample, in association with a plurality of target awake time interval(or a plurality of slots constructing RAW), the channel accesspermission interval of the STA group based on GID may be allocated inunits of a target awake time interval (or slot).

FIG. 20 is a conceptual diagram illustrating an exemplary target awaketime (TAT) information element format according to another embodiment.

Referring to FIG. 20, a target awake time (TAT) time IE may furtherinclude an Uplink Channel Access Control field and an Uplink ChannelAccess Duration field in addition to the example of FIG. 18. Using thetarget awake time IE shown in FIG. 20, the AP may control the CCA ruleapplied to the STA configured to perform channel access at a targetawake time (TAT).

In more detail, the uplink channel access control field of FIG. 20 maybe used as a specific value indicating whether TXOP can overlap (orcross) with a target awake time (TAT). For example, if an uplink channelaccess control field is set to 1 (or true), TXOP, that is in progressjust before the target awake time (TAT), must stop operation before thetarget awake time (TAT) starts. That is, the TXOP need not extend crossthe target awake time (TAT) (i.e., TXOP need not overlap the targetawake time (TAT). In this case, if STA transitions to the awake state ata target awake time (TAT), it is not necessary to perform the mediumsynchronization through the CCA process. That is, the STA switched fromthe doze state to the awake state at a target awake time (TAT) does notperform the CCA operation (i.e., until a frame sequence is not detectedor without waiting for the lapse of ProbeDelay), and the STA canimmediately perform the channel access operation. In other words, ifTXOP extending across the target awake time (TAT) is prevented frombeing used, the STA awakened at the n-th target awake time (or the n-thslot boundary) can obtain channel access (or TXOP) only at a targetawake time TAT (or a slot) reaching the (n+1)-th target awake time (orthe (n+1)-th slot boundary).

Alternatively, if the uplink channel access control field is set to zero(false), TXOP extending across the target awake time (TAT) is allowed(i.e., the TXOP can overlap (or cross) with the target awake time (TAT).That is, TXOP can be used irrespective of the target awake time (TAT).In this case, the STA switched from the doze state to the target awaketime (TAT) state needs to perform medium synchronization upon completionof CCA during a predetermined time. For example, a maximum value of thepredetermined time may be denoted by a ProbeDelay value.

The uplink channel access time duration field may include specificinformation for indicating limitation of a specific time (e.g., TXOP) inwhich STA(s) can be used. For example, the uplink channel accessduration field may include information of a maximum time in which STA(s)can use the channel after lapse of a target awake time (TAT). In thiscase, the target awake STA (i.e., STA in which channel access is allowedat the corresponding TAT) starts the random access process starts at atarget awake time (TAT) time offset (See FIG. 18), obtains TXOP, and canuse the TXOP during the uplink channel access time. Alternatively, theuplink channel access duration field may also be set to a specific valueindicating whether the TXOP length exceeds the target awake timeinterval (TAT) field (or the slot length). The above-mentioned methodmay be considered an additional method for indicating whether TXOPextends across the target awake time (TAT) or (a slot boundary) at theuplink channel access control field.

FIG. 21 is a flowchart illustrating a channel access operation for usein the case in which a plurality of target awake times is established.

FIG. 21 assumes that a plurality of target awake times (TATs) isestablished in one beacon frame as shown in FIG. 19. Therefore, as shownin FIG. 19, a plurality of TATs (or a slot boundary) is established inone beacon frame, and one or more TAT interval (or one or more slots)may be established. In the example of FIG. 21, N is a Null Data Packet(NDP) frame, D is a data frame, and A is an ACK frame. The NDP framewill hereinafter be described with reference to FIG. 22.

The STA in which channel access is allowed at a target awake time (TAT)as shown in FIG. 21 is shifted to the awake state, and can transmit theUL data frame through the channel access process. In this case, STAswitching from the target awake time (TAT) to the awake stateimmediately performs the backoff process without the mediumsynchronization process (or without performing CCA, and then performschannel access. In the example of FIG. 21, STA in which channel accessis allowed at a target awake time (TAT) transmits the NDP frame (e.g.,NDP PS-Poll) through contention based on the backoff process, such thatthe STA receives the data frame and then transmits the ACK frame.

In addition, if the uplink channel access control field (See FIG. 20) isset to 1, TXOP cannot overlap (cross) with the target awake time (TAT),TXOP must expire before the target awake time (TAT). FIG. 21 shows thatTXOP (i.e., transmission or reception of data, transmission or receptionof ACK) configured to exchange the frame on WM of the STA is completedbefore the next target awake time (TAT).

NDP Frame

FIG. 22 is a conceptual diagram illustrating an NDP frame formataccording to an exemplary embodiment.

FIG. 22( a) shows a legacy basic Physical Layer Convergence Protocol(PLCP) Packet Data Unit (PPDU) frame format.

The legacy PPDU frame format may include a Short Training Field (STF), aLong Training Field (LTF), a signal (SIG) field, and a data field. Themost basic (for example, non-HT) PPDU frame format may be comprised of aLegacy-STF (L-STF) field, a Legacy-LTF (L-LTF) field, an SIG field, anda data field. In addition, the most basic PPDU frame format may furtherinclude additional fields (i.e., STF, LTF, and SIG field) between theSIG field and the data field according to the PPDU frame format types(for example, HT-mixed format PPDU, HT-greenfield format PPDU, a VHTPPDU, and the like).

STF is a signal for signal detection, Automatic Gain Control (AGC),diversity selection, precise time synchronization, etc. LTF is a signalfor channel estimation, frequency error estimation, etc. The sum of STFand LTF may be referred to as a PCLP preamble. The PLCP preamble may bereferred to as a signal for synchronization and channel estimation of anOFDM physical layer.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may include information regarding data modulation and coding rate.The LENGTH field may include information regarding the length of data.Furthermore, the SIG field may include a parity field, a SIG TAIL bit,etc.

The data field may include a service field, a PLCP Service Data Unit(PSDU), and a PPDU TAIL bit. If necessary, the data field may furtherinclude a padding bit. Some bits of the SERVICE field may be used tosynchronize a descrambler of the receiver. PSDU may correspond to a MACPDU (Protocol Data Unit) defined in the MAC layer, and may include datagenerated/used in a higher layer. A PPDU TAIL bit may allow the encoderto return to a state of zero (0). The padding bit may be used to adjustthe length of a data field according to a predetermined unit.

FIG. 22( b) exemplarily shows the legacy PS-Poll frame format.

Referring to FIG. 22( b), the legacy PS-Poll frame is defined as the MACframe format, and may correspond to a control frame according to framecategories. The MAC frame may be composed of a MAC header, a frame body,and a Frame Check Sequence. The MAC frame is composed of MAC PDUs, suchthat it can be transmitted or received through PSDU of a data part ofthe PPDU frame format of FIG. 22( a).

In the example of FIG. 22( b), the PS-Poll frame format may be comprisedof a frame control field, an AID field, a BSSID (RA (ReceivingAddressing)) field, a transmitting address (TA), and an FCS field. Theframe control field may include control information needed for frametransmission/reception. The AID field may have an AID value allocated tothe STA configured to transmit the PS-Poll frame. The BSSID(RA) fieldmay correspond to the AP address, and the TA field may correspond to anaddress of the STA configured to transmit the frame. In this case, theMAC header may be comprised of the frame control field, the AID field,the BSSID(RA) field, and the TA field. That is, the PS-Poll frame formatmay be comprised of the MAC header and the FCS only without inclusion ofthe frame body.

The frame control field may include a protocol version field, a Typefield, a Subtype field, a To DS field, a From DS field, a More Fragment(MF) field, a Retry field, a Power Management (PM) field, a More Data(MD) field, a Protected Frame (PF) field, and an Order field.

FIG. 22( c) shows the NDP frame format proposed by the presentinvention. The NDP frame may represent the frame structure having nodata packet. That is, the NDP frame may represent a frame format thatincludes the initial PLCP preamble part and the SIG field of FIG. 18( a)without inclusion of the remaining parts (i.e., data field). Inassociation with a frame transmitted from the STA to the AP and anotherframe transmitted from the AP to the STA for channel access, the NDPframe format shown in FIG. 22( c) is used, so that the embodiment of thepresent invention can reduce a delay time while simultaneously reducingpower consumption of the STA.

For example, the STA may use the NDP frame proposed by the presentinvention, instead of using the PS-Poll frame to be transmitted to theAP. That is, whereas the legacy PS-Poll frame is identical to the MACcontrol frame that is transmitted through PSDU of the data field of FIG.22( a), the present invention may use the NDP PS-Poll including no PDSUas necessary.

In more detail, the STA switching to the awake state at a target awaketime (TAT) established by the AP for a specific STA can perform channelaccess without performing CCA. The following NDP PS-Poll frame may beused as the NDP frame transmitted through the backoff process in FIG.21.

In the case in which the NDP frame format is configured as shown in FIG.22( c), the data field (e.g., MAC control frame of FIG. 22( b)) of FIG.22( a) is not included in the NDP frame format, so that informationcorresponding to the frame control field of PSDU (i.e., MAC frame) isnot contained therein. However, a minimum amount of control informationmust be contained in the NDP frame so as to transmit/receive the NDPframe. For this purpose, the present invention provides a method forincluding the above-mentioned information in the SIG field of FIG. 22(c).

That is, as described above, the NDP frame may include STF, STF, and SIGfield only. In this case, each of STF and LTF may be composed of achannel estimation signal (or sequence) needed for decoding the SIGfield. The SIG filed may include a plurality of subfields. For example,the SIG field may include a Type subfield, an AID subfield, a BSSIDsubfield, a Cyclic Redundancy Check (CRC) subfield, etc. In addition,the SIG field may include additional subfields as well as the foursubfields, and the order of subfields is only exemplary rather thanrestrictive.

The Type subfield is used to interpret the SIG field of the NPD frame,and may be set to a specific value indicating the usage of thecorresponding NDP frame. For example, if the Type field has apredetermined value, this may indicate that the corresponding NDP frameis an NDP PS-Poll frame. In other words, the SIG field of the NDP framemay be a modified SIG field different from the legacy SIG field (i.e.,composed of the RATE field and the LENGTH field, and may indicatewhether the corresponding SIG field is a legacy SIG field or a modifiedSIG field using the Type field acting as a first subfield of the SIGfield.

The AID subfield may correspond to the SID of the STA configured totransmit the NDP frame. The AID subfield may be configured to indicate agroup ID (or PID). In addition, the AID subfield may correspond to aPartial AID (PAID) defined as an abbreviated form of AID. In addition,the AID subfield may correspond to a predetermined ID value (e.g., a newAID format, or a resultant value obtained by hashing the legacy AID) foridentifying the corresponding STA. The AP having receiving the NDPPS-Poll frame may recognize which STA has been used for transmission ofthe PS-Poll frame on the basis of AID (or PAID).

The BSSID subfield may correspond to a BSSID of the AP including the STAhaving transmitted the NDP frame. In addition, the BSSID subfield maycorrespond to a Partial BSSID (PBSSID) defined as an abbreviated formatof the BSSID. In addition, the BSSID subfield may correspond to apredetermined ID value (e.g., a new AID format, or a resultant valueobtained by hashing of the legacy AID) for identifying the correspondingAP. The BSSID subfield may indicate a reception address (RA) of the NDPframe.

The CRC subfield may be used to detect errors of the SIG field of theNDP frame.

A method for enabling the STA to transmit the NDP PS-Poll frame usingthe above-mentioned NDP frame according to the present invention willhereinafter be described in detail.

STA configured to perform channel access at a target awake time (TAT)switches the awake state and the transmits the NDP PS-Poll frame withinthe target awake time (TAT) interval. The AP having received the NDPPS-Poll frame may determine whether the AP must answer the NDP PS-Pollframe through the BSSID (or PBSSID) subfield contained in the SIG field.As a response to the NDP PS-Poll frame, the AP may transmit the ACKframe or may transmit the buffered data frame for the corresponding STA.

The case in which the AP transmits the ACK frame may indicate anexemplary case in which the buffered data for the corresponding STA isnot present or it is difficult to immediately transmit the data frameafter lapse of SIFS upon receiving the NDP PS-Poll frame irrespective ofthe presence or absence of the buffered data. If the buffered data forthe STA is not present, the More Data (MD) bit of the frame controlfield of the ACK frame transmitted from the AP to the STA may be set tozero 0. Alternatively, when using the case in which the ACK frame istransmitted irrespective of the presence or absence of buffered data forthe STA, the MD bit may be set to 1.

FIG. 23 is a flowchart illustrating the channel access method accordingto an exemplary embodiment.

Referring to FIG. 23, a first STA (e.g., AP) may provide a second STA(e.g., non-AP STA) with configuration information regarding one or moreslots (or one or more TAT intervals) and one or more slot boundaries (orone or more TATs). In addition, the above slot configuration informationmay include specific information indicating whether TXOP overlaps withthe slot boundary (or TAT), or may include specific informationindicating whether TXOP extends access the slot boundary. In FIG. 23,TXOP is prevented from overlapping with the slot boundary (or TAT).

In steps S2330 to S2330, the second STA switches to the awake state at aslot boundary (or TAT) established by the first STA, and can performchannel access. In more detail, since slot boundary overlapping of theTXOP of step S2310 is prevented in step S2320, the STA can immediatelystart the channel access in step S2330 without performing CCA. Thechannel access start may include a process for transmitting the framethrough contention (through the backoff process). Here, the frame may bean NDP PS-Poll frame. Accordingly, the NDP frame having a minimum lengthis transmitted at a slot in which channel access is allowed withoutperforming CCA, such that power consumption of the second STA can beminimized.

The above described various embodiments of the present invention may beindependently applied or two or more embodiments thereof may besimultaneously applied.

FIG. 24 is a block diagram illustrating a radio frequency (RF) deviceaccording to an embodiment of the present invention.

Referring to FIG. 24, an AP 10 may include a processor 11, a memory 12,and a transceiver 13. An STA 20 may include a processor 21, a memory 22,and a transceiver 13. The transceivers 13 and 23 may transmit/receiveradio frequency (RF) signals and may implement a physical layeraccording to an IEEE 802 system. The processors 11 and 21 are connectedto the transceivers 13 and 21, respectively, and may implement aphysical layer and/or a MAC layer according to the IEEE 802 system. Theprocessors 11 and 21 can be configured to perform operations accordingto the above-described embodiments of the present invention. Modules forimplementing operation of the AP and STA according to the abovedescribed various embodiments of the present invention are stored in thememories 12 and 22 and may be implemented by the processors 11 and 21.The memories 12 and 22 may be included in the processors 11 and 21 ormay be installed at the exterior of the processors 11 and 21 to beconnected by a known means to the processors 11 and 21.

The overall configuration of the AP and STA may be implemented such thatabove described various embodiments of the present invention may beindependently applied or two or more embodiments thereof may besimultaneously applied and a repeated description is omitted forclarity.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

Although the above various embodiments of the present invention havebeen described based on an IEEE 802.11 system, the embodiments may beapplied in the same manner to various mobile communication systems.

1. A method for performing channel access by a station (STA) of awireless communication system, the method comprising: receivingconfiguration information regarding at least one slot in which channelaccess of the STA is allowed, from an access point (AP); and startingchannel access at a slot boundary of the at least one slot, wherein thechannel access starts without performing of Clear Channel Assessment(CCA).
 2. The method according to claim 1, wherein: the configurationinformation regarding the at least one slot further includes specificinformation indicating whether Transmission Opportunity (TXOP) of theSTA is allowed to overlap the slot boundary, and if the TXOP is notallowed to overlap the slot boundary, the channel access starts withoutperforming the CCA.
 3. The method according to claim 2, wherein the TXOPis obtained in one slot duration from among the at least one slot. 4.The method according to claim 2, wherein the STA is switched from a dozestate to an awake state.
 5. The method according to claim 1, wherein theCCA is performed until a frame sequence for the STA to configure anetwork allocation vector (NAV), or until a period of time equal to aProbeDelay value has transpired.
 6. The method according to claim 1,wherein the slot boundary is a time point at which channel access of theSTA is allowed.
 7. The method according to claim 6, wherein the slot isan interval between two contiguous time points.
 8. The method accordingto claim 6, wherein the time point at which the channel access isallowed is a target awake time of the STA.
 9. The method according toclaim 1, wherein the starting the channel access includes transmitting aframe through contention.
 10. The method according to claim 9, whereinthe frame is an NDP Power Save (PS)-Poll frame.
 11. The method accordingto claim 1, wherein a plurality of slots is configured during a beaconinterval.
 12. The method according to claim 1, wherein the configurationinformation of the at least one slot is provided through a beacon frame.13. The method according to claim 1, wherein the configurationinformation of the at least one slot is configuration information of atime duration in which restricted channel access of the STA is allowed.14. The method according to claim 1, wherein the configurationinformation of the at least one slot includes information for allocatingthe channel access duration allowed to a STA group including the STA ona slot basis.
 15. A station (STA) device configured to perform channelaccess in a wireless communication system, comprising: a transceiver;and a processor, wherein the processor is configured to receive, throughthe transceiver, configuration information regarding at least one slotin which channel access of the STA is allowed, from an access point(AP), and to start channel access at a slot boundary of the at least oneslot, wherein the channel access starts without execution of ClearChannel Assessment (CCA).